CSCI312 Principles of Programming Languages!

Transcription

1 CSCI312 Principles of Programming Languages! Chapter 5 Types Xu Liu!

2 ! 5.1!Type Errors! 5.2!Static and Dynamic Typing! 5.3!Basic Types! 5.4!NonBasic Types! 5.5!Recursive Data Types! 5.6!Functions as Types! 5.7!Type Equivalence! 5.8!Subtypes! 5.9!Polymorphism and Generics! 5.10!Programmer-Defined Types

3 A type is a collection of values and operations on! those values. Example: Integer type has values..., -2, -1, 0, 1, 2,... and operations +, -, *, /, <,... The Boolean type has values true and false and operations,,.

4 Computer! types have a finite number of values due to fixed size allocation; problematic for numeric types. Exceptions: Smalltalk uses unbounded fractions. Haskell type Integer represents unbounded integers. Floating point problems?

5 ! Even more problematic is fixed sized floating point numbers: 0.2 is not exact in binary. So 0.2 * 5 is not exactly 1.0 Floating point is inconsistent with real numbers in mathematics.

6 ! In the early languages, Fortran, Algol, Cobol, all of the types were built in. If needed a type color, could use integers; but what does it mean to multiply two colors. Purpose of types in programming languages is to provide ways of effectively modeling a problem solution.

7 5.1 Type Errors! Machine data carries no type information. Basically, just a sequence of bits. Example:

8 ! The floating point number The 32-bit integer 1,079,508,992 Two 16-bit integers and 0 Four ASCII X NUL NUL!

9 A! type error is any error that arises because an operation is attempted on a data type for which it is undefined. Type errors are common in assembly language programming. High level languages reduce the number of type errors. A type system provides a basis for detecting type errors.

10 5.2 Static and Dynamic Typing! A type system imposes constraints such as the values used in an addition must be numeric. Cannot be expressed syntactically in EBNF. Some languages perform type checking at compile time (eg, C). Other languages (eg, Python) perform type checking at run time. Still others (eg, Java) do both.

11 ! A language is statically typed if the types of all variables are fixed when they are declared at compile time. A language is dynamically typed if the type of a variable can vary at run time depending on the value assigned. Can you give examples of each?

12 A language is strongly typed if its type system allows all type errors in a program to be detected either at compile time or at run time. A strongly typed language can be either statically (e.g., Ada, Java) or dynamically typed (e.g., Python and Perl).

13 An Example C Program int main(){ union {int a; float p;} u; u.a = -1; float x=0; x = x + u.p; printf ("x=%f\n", x); float y=0; y = y + (float)u.a; printf ("y=%f\n", y); return 0; } u.a u.p x = -nan y = Union makes a hole in type checking. C/C++ are not strongly typed.

14 5.3 Basic Types! Terminology in use with current 32-bit computers: Nibble: 4 bits Byte: 8 bits Half-word: 16 bits Word: 32 bits Double word: 64 bits Quad word: 128 bits

15 ! In most languages, the numeric types are finite in size. So a + b may overflow the finite range. Unlike mathematics: a + (b + c) (a + b) + c! Also in C-like languages, the equality and relational operators produce an int, not a Boolean

16 Overloading An operator or function is overloaded when its meaning varies depending on the types of its operands or arguments or result. Python: a + b (ignoring size) integer add floating point add string concatenation What if a is integer while b is floating point?

17 Type Conversion A type conversion is a narrowing conversion if the result type permits fewer bits, thus potentially losing information. E.g., float -> int Otherwise it is termed a widening conversion. E.g., int -> float Explicit conversion: 2 + int(1.3); Implicit conversion: ; Should languages use narrowing or widening for implicit conversions?

18 5.4 Nonbasic Types! Enumeration: enum day {Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday};! enum day myday = Wednesday;! In C/C++ the above values of this type are 0,..., 6. More powerful in Java: for (day d : day.values())!!sytem.out.println(d);!

19 Pointers! C, C++, Ada, Pascal Java??? Value is a memory address Indirect referencing Operator in C: *

20 Pointer Operations If T is a type and reft is a pointer: & : T reft. Eg. &x: returns the address of x * : reft T. Eg. *p: returns the value in the location that p references. For an arbitrary variable x: *(&x) = x

21 Example int main(){ int v = 3; int * p = &v; (*p) = -3; printf("v=%d\n", v); return 0; }

22 Pointers are convenient in some cases Example: Linked List struct Node {! int key;! struct Node* next; }; struct Node* head;

23 But Error-Prone E.g. Buffer overflow problem String copy: while (*p++ == *q++); q points to a string$ p points to a 3-char buffer. Particularly troublesome in C as points and array are regarded the same.

24 Equivalence between arrays and pointers int a[100]; // declare an array a == &a[0] a[i] == *(a + i) float sum(float a[ ], int n) { int i; float s = 0.0; for (i = 0; i<n; i++)!!!! s += a[i]; return s; float sum(float *a, int n) { int i; float s = 0.0; for (i = 0; i<n; i++)! s += *a++; return s;

25 Arrays and Lists int a[10]; float x[3][5]; /* odd syntax vs. math */ char s[40];

26 Indexing The only operation for arrays and lists in many languages Type signature [ ] : T[ ] int T Example float x[3] [5]; type of x: float[ ][ ] type of x[1]: float[ ] type of x[1][2]: float

27 Strings Now so fundamental, directly supported. In C, a string is a 1D array with the string value terminated by a NUL character (value = 0). In Java, Perl, Python, a string variable can hold an unbounded number of characters. Libraries of string operations and functions.

28 Structures Analogous to a tuple in mathematics Collection of elements of different types Used first in Cobol, PL/I Absent from Fortran, Algol 60 Common to Pascal-like, C-like languages Omitted from Java as redundant

29 struct employeetype { int id; char name[25]; int age; float salary; char dept; }; struct employeetype employee;... employee.age = 45;

30 Unions C: union Pascal: case-variant record union {int a; float p;} u; Logically: multiple views of same storage Useful in some systems applications

31 Contents 5.1! Type Errors 5.2! Static and Dynamic Typing 5.3! Basic Types 5.4! NonBasic Types 5.5! Recursive Data Types 5.6! Functions as Types 5.7! Type Equivalence 5.8! Subtypes 5.9! Polymorphism and Generics 5.10!Programmer-Defined Types

32 5.5 Recursive Data Type Example: Linked List struct Node {! int key;! struct Node* next; }; struct Node* head; Others?

33 5.6 Functions as Types Needs example: a function to draw the curve for y=f(x). Pascal example:! function newton(a, b: real; function f: real): real; Know that f returns a real value, but the arguments to f are unspecified.

34 Addressed by Java Interface public interface RootSolvable { double valueat(double x); } public double Newton(double a, double b, RootSolvable f);

35 5.7 Type Equivalence Pascal Report: The assignment statement serves to replace the current value of a variable with a new value specified as an expression.... The variable (or the function) and the expression must be of identical type. Nowhere does it define identical type.

36 struct complex { float re, im; }; struct polar { float x, y; }; struct { float re, im; } a, b; struct complex c, d; struct polar e; int f[5], g[10]; // which are equivalent types?

37 Kinds of Type Equivalence Name equivalence Structural equivalence: # and order of fields of a structure, and the name and type of each field. Ada, Java: name equivalence. C: name equivalence for structs and unions, structural equivalence for other constructed types (arrays and pointers). Size of an array doesn t matter.

38 struct complex { float re, im; }; struct polar { float x, y; }; struct { float re, im; } a, b; struct complex c, d; struct polar e; int f[5], g[10]; // which are equivalent types? Name equivalence: a, b Structural equivalence: a, b, c, d f, g are equivalent in both cases.

39 5.8 Subtypes A subtype is a type that has certain constraints placed on its values or operations. In Ada subtypes can be directly specified.

40 subtype one_to_ten is Integer range ; type Day is (Monday, Tuesday, Wednesday, Thursday, Friday, Saturday, Sunday); subtype Weekend is Day range Saturday.. Sunday; type Salary is delta 0.01 digits 9 range _999_999.99; subtype Author_Salary is Salary digits 5 range ;

41 Example in Java Integer i = new Integer(3);... Number v = i;... Integer x = (Integer) v; //Integer is a subclass of Number, // and therefore a subtype

42 Polymorphism and Generics A function or operation is polymorphic if it can be applied to any one of several related types and achieve the same result. An advantage of polymorphism is that it enables code reuse.

43 Polymorphism Comes from Greek Means: having many forms Example: overloaded built-in operators and functions + - * / ==!=... Java: + also used for string concatenation

44 Ada, C++: define for new types Java overloaded methods: number or type of parameters Example: class PrintStream print, println defined for: boolean, char, int, long, float, double, char[ ], String, Object

45 Java: instance variable, method name, name( ) Ada generics: generic sort parametric polymorphism type binding delayed from code implementation to compile time procedure sort is new generic_sort(integer);

46 generic type element is private; type list is array(natural range <>) of element; with function ">"(a, b : element) return boolean; package sort_pck is procedure sort (in out a : list); end sort_pck;

47 package sort_pck is procedure sort (in out a : list) is begin for i in a'first.. a'last - 1 loop for j in i+1.. a'last loop if a(i) > a(j) then declare t : element; begin t := a(i); a(i) := a(j); a(j) := t; end; end if;

48 Instantiation package integer_sort is new generic_sort( Integer, ">" );

49 Programmer-defined Types Recall the definition of a type: A set of values and a set of operations on those values. Structures allow a definition of a representation; problems: Representation is not hidden Type operations cannot be defined Defer further until Chapter 12.